This invention generally pertains to nozzles for injection molding. More specifically, the present invention relates to a nozzle for injection molding of plastic materials, which nozzle utilizes fluid assist.
The invention is particularly applicable to a gas assisted injection molding nozzle which enables a viscous fluid, such as a molten plastic, to selectively flow into a mold cavity and also enables the selective flow of a non-viscous fluid, such as a gas, into the mold cavity. However, it will be appreciated by those skilled in the art that the invention has broader applications and may also be adapted for use in many other injection molding environments where both a relatively viscous fluid, such as a plastic or wax, and a relatively non-viscous fluid, such as a gas or liquid, are injected into a mold cavity.
Recently, gas assisted injection molding has gained popularity. In this process, the mold cavity is filled with a plasticized thermoplastic material, usually to a volume less than 100% of the mold cavity, and an inert gas is injected under pressure into the plasticized material to fill the rest of the volume in the mold cavity. The gas is injected into the center of the flow of plastic but does not mix with the melt and instead runs along specially designed channels. In this way, with a suitably designed part, a continuous network of hollowed out sections can be provided. The material displaced by the gas from the middle of the sections moves out to fill the remainder of the mold space.
This network of gas channels provides a uniform pressure distribution system throughout the mold space during part rehardening and cool down thus minimizing internal stresses in the part. Gas injection provides a solution to a number of problems that have long plagued the injection molding industry. These include reduction of stress and warpage of the plastic part, elimination of sink marks and the provision of smooth surfaces on the injection molded part. In addition, clamp tonnage requirements can be reduced in comparison to conventional injection molding processes. The process also permits different wall thicknesses for a single part and faster cycle times in comparison with the conventional injection molding processes. Also, gas assisted injection molding reduces the need for external flow runners.
Several types of nozzles are known for gas assisted injection molding. However, many of these nozzles do not vent the gas back through the nozzle when the discharge of the gas is required. Even those nozzles which do vent the gas back through the nozzle are unsatisfactory because the molten plastic remaining in the nozzle or in the sprue and runner system is frequently vented back along with the gas thus causing one of the major difficulties with gas assisted injection molding, namely the plugging of gas channels in the nozzle with thermoplastic which solidifies and blocks off further gas flow through these channels. In addition, the gas piping and valves downstream from the nozzle can become plugged. The nozzle then becomes unusable until it is cleaned out which is a time consuming, difficult and expensive process.
A recently perfected nozzle does away with many of these disadvantages through the use of a pin which remains stationary as a barrel of the nozzle reciprocates around the pin. However, this type of apparatus requires the use of seal means to prevent the flow of gas through the barrel in a direction away from the mold cavity. As the barrel reciprocates in relation to the stationary pin, the seal means quickly becomes worn and begins to leak thus requiring a disassembly of the nozzle to allow a replacement of the seal means. The type of pin employed in the known device is also rather long and may be prone to breakage if there is a malfunction of the barrel reciprocating mechanism.
Accordingly, it has been considered desirable to develop a new and improved gas assisted injection molding nozzle which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.